Power converters and in particular switched-mode power converters are used in a variety of applications to provide AC/DC and DC/DC conversion. For example, switched-mode power converters, also referred to as switched-mode power supplies (SMPS), are widely used in computer and mobile phone power supply units to provide the necessary operating voltages from typical 120V/240V AC mains lines.
Typical items of concern when designing power converters relate to conversion efficiency and cost. It should be readily apparent that power losses should be minimized to increase the overall efficiency of the converter and also to reduce the generation of heat, which may be difficult to dissipate depending on the design and the respective application.
It is known to operate switched-mode power converters in boundary conduction mode or short “boundary mode” (BCM). Unlike a continuous operation in CCM (continuous conduction mode), in boundary conduction mode it is aimed to operate the switch of the power converter when no or no substantial current flows through the switch. This operational mode reduces switching losses and also allows to use less expensive components, for example less expensive boost diodes in a boost switched-mode power converter setup due to no reverse-recovery losses. In addition, BCM also allows for power factor correction (PFC), in view that the input current follows the input voltage waveform.
A by-product of BCM is that the converter inherently uses a variable switching frequency. The frequency depends primarily on the selected output voltage, the instantaneous value of the input voltage, the parameters of the energy storage used, e.g., inductance or capacitance, and the output power delivered to the load. The lowest frequency occurs at the peak of sinusoidal line voltage, the highest frequency near the zero-crossings of the sinusoidal line voltage.
Interleaved power converters are used when higher currents are to be converted. These types of power converters comprise multiple stages/circuits, typically arranged in parallel to each other. The term “interleaved” in the present context means that multiple circuits are operated out of phase. For example, in an interleaved power converter having two stages, the stages typically operate at 180 degrees out of phase, i.e., half the switching cycle out of phase. Interleaved power converters bear the advantage of causing less input current variation/ripple and thus cause less electromagnetic interference issues.
A problem when operating an interleaved power converter in boundary conduction mode resides in the fact that the input voltage is very small near the zero-crossing points. Near the zero-crossing points, the input energy is not transferred to output side as it is spent in charging internal device parasitics, such as for example capacitances of MOSFET switches. Further due to the high switching frequency near the zero-crossing points of the input voltage, typical filters for electromagnetic interference (EMI) attenuate such high frequency components, resulting in a prolonged zero current situation near the zero-crossing points and a distorted current waveform.